It is chemically and clinically important to do qualitative and quantitative analyses of materials contained in a bio sample, including, for example, blood glucose for diabetic patients or cholesterol in blood that is a factor of many adult diseases.
As known in the art, measuring enzymatic activity for a specific material contained in a bio sample (hereinafter, referred to as “sample”) more rapidly with good reproducibility is very important to the electrochemical biosensor using enzymatic activity such as, for example, glucose sensor, uric acid sensor, protein sensor, DNA sensor or sucrose sensor for clinical chemical tests, or GOT (Glutamate-Oxaloacetate Transaminase) or GPT (Glutamate-Pyruvate Transaminase) sensor for liver function test.
The biosensor is composed of an identification part for identifying an analyte target, and a conversion part that is in charge of conversion to electrical signals. Biomaterials are used as the identification part of the biosensor. The biomaterial recognizes an analyte target to cause a chemical or physical change, and the conversion part converts the chemical or physical change to electrical signals. The identification part and the conversion part are collectively called “biosensor electrode”.
Generally, the measurement method using the conventional strip type biosensor involves inserting a sample into a sample insertion path by way of capillary action, which is stronger than terrestrial gravity and acquired through plasma or chemical surfactant treatment in the course of fabrication, accumulating the sample in the sample insertion path and then doing a qualitative and quantitative analysis of the sample.
The measurement method normally further includes, prior to the sample measurement, detecting a sample insert timing that is when the inserted sample begins to accumulate in the sample insertion path.
Conventionally, a sample insertion detection signal is applied to a working electrode and a reference electrode of the biosensor to detect the sample insert timing and, after an elapse of predetermined time, a sample measurement signal is applied to the working electrode and the reference electrode to measure the sample. In other words, the sample insert timing is determined by calculating the time taken to make the sample completely inserted into the sample insertion path in all volume from the arrival time of the sample at the working electrode and the reference electrode. Then, a predetermined time later, the sample measurement signal is applied to the working electrode and the reference electrode to take a measurement of the sample.
This case encounters a problem that the sample insertion detection signal applied to detect the sample insert timing is too early reactive to the sample on the surface of the working electrode and the reference electrode, which are important as electrodes for measurement, to form an electric double layer (EDL). The electric double layer, which usually appears on the interface between adjacent different substances (electrode, sample, or solution), is formed when electric field is applied to the interface. The capacitance of the electric double layer, which is called “double layer capacitance (DLC)”, is of a meager amount but can be included in the current signal measured when the sample detection signal is applied. This causes a distortion of the measurement signal of the biosensor and affects the measurement results.
The viscosity of the sample depends on the type of the sample and, if not significant, may determine the sample's accumulating velocity or time in the sample insertion path. This may also affect the measurement results because the measurement begins after an elapse of predetermined time that is calculated as the time taken to accumulate the sample completely into the path from the sample insert timing. It is therefore considerably problematic in regard to accuracy of measurement to determine the timing to apply the sample measurement signal by control of timing.